Fluid bed desulfurization using a zinc-based metallic oxide sorbent is known in the art. Typically, a fuel gas produced by gasification of fossil fuels contains sulfur compounds which must be removed before the fuel gas can be utilized. To effect sulfur clean-up, the sulfur-containing fuel gas is contacted with a particulated metallic oxide sorbent at an elevated temperature. These gas-solids reaction systems conventionally employ a fixed or fluid bed reactor. Reaction between the sulfur and the sorbent desulfurizes the fuel gas and produces a spent sulfided sorbent. The spent sorbent is then regenerated for sulfur absorption by reaction with an oxidant gas, e.g. air at an elevated temperature to produce a sulfur dioxide-containing offgas. Typically, the offgas is then reacted with lime to form sulfate to complete the sulfur clean-up.
Metallic oxide sorbents are known to undergo attrition at elevated temperatures as the chemical and physical sorbent structure becomes degraded. To avoid excessive sorbent degradation through repeated absorption-regeneration cycles, the reaction temperature of the sulfur absorption and sorbent regeneration reactions must generally be held below about 650.degree. C.-760.degree. C. Control of the sulfur absorption reaction temperature has not generally been a problem since the sulfur absorption reaction generates only a small amount of heat. However, control of the regeneration reaction temperature is more difficult because the sulfur regeneration reaction is highly exothermic. To avoid an excessive regeneration reaction temperature which can damage the sorbent, the regeneration reaction rate must be limited. Generally, this entails the use of a diluent gas such as nitrogen and/or steam in the oxidant gas to absorb generated heat and lower oxidant (oxygen) concentration, a sorbent cooler or a combination of both.
There are several drawbacks to the use of a diluent gas in sorbent regeneration. A diluent gas increases the volumetric throughput in the regenerator reactor. Vessel size of the regenerator train must be increased in order to maintain a desired residence time. Sulfur dioxide concentration of the offgas becomes too low for further high value-added application such as the manufacture of sulfuric acid and/or reduction to sulfur. Use of a sorbent cooler is undesirable due to additional capital costs involved.
It would be advantageous to operate a zinc-based metallic oxide desulfurization process in a manner, which substantially reduces or eliminates the need for a diluent or for sorbent cooling in the sorbent regeneration mode, and which produces an offgas having a sulfur dioxide concentration that can be suitable for use in sulfuric acid manufacture and/or reduction to sulfur.
Ayala et al., "Enhanced Durability of High-Temperature Desulfurization Sorbents for Moving-Bed Applications," GE Corporate Research and Development, Schenectady, N.Y., May 1992, DE-AC21-88MC25003 describes the development of chemically active and mechanically durable zinc ferrite and zinc titanate sorbent formulations which are suitable for moving-bed, high temperature, coal gas desulfurization processes.
Morgantown Energy Technology Center, "Fluid-Bed Hot-Gas Desulfurization Process Development Unit," U.S. Department of Energy, describes the use of a fluid-bed hot gas desulfurization (HGD) process development unit (PDU) in an integrated gasification combined cycle (IGCC) system involving the continuous circulation of a desulfurization sorbent such as zinc titanate, zinc ferrite or other materials between an absorber and a regenerator. In the absorber, the sorbent becomes sulfided as a result of removing sulfur species from a fuel gas stream. In the regenerator, the captured sulfur in the sulfided sorbent is oxidized with air to restore the sorbent activity and yield SO.sub.2.